The Effect of Laser Scanning Strategies on Texture, Mechanical Properties, and Site-Speciﬁc Grain Orientation in Selective Laser Melted 316L SS

Materials and Design 193 (2020) 108852 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes The effect of laser scanning strategies on texture, mechanical properties, and site-specific grain orientation in selective laser melted 316L SS Jithin James Marattukalam a,⁎, Dennis Karlsson b, Victor Pacheco b, Přemysl Beran c,e, Urban Wiklund d, Ulf Jansson b, Björgvin Hjörvarsson a, Martin Sahlberg b a Department of Physics, Materials Physics, Uppsala University, Box 530, SE-75121 Uppsala, Sweden b Department of Chemistry- Ångström Laboratory, Uppsala University, Box 523, 75120 Uppsala, Sweden c Department of Neutron Physics, Nuclear Physics Institute, ASCR, CZ -25068 Řež, Czech Republic d Department of Engineering Sciences, Applied Materials Science, Uppsala University, Box 534, 75121 Uppsala, Sweden e European Spallation Source ERIC, Box 176, 22100 Lund, Sweden HIGHLIGHTS GRAPHICAL ABSTRACT • Laser scanning strategies strongly influence the crystallographic texture in as- printed components. • A ⟨100⟩ single crystalline-like texture is obtained in the direction of laser writ- ing. • Site-specific texture control is achieved by selectively switching laser scanning strategies. article info abstract Article history: Selective laser melting has been used to demonstrate the striking effect of laser scanning strategies on the crys- Received 13 December 2019 talline texture in 316L SS. The aligned crystal orientation along the tensile direction (Z-axis) could be varied using Received in revised form 1 June 2020 the scanning strategy. A strong 〈100〉 single crystalline-like texture is obtained in the direction of the laser scan Accepted 2 June 2020 and a 〈110〉 texture was observed in the build direction when using a bidirectional scan without rotation. Fiber Available online 03 June 2020 texture was observed along the tensile direction when the bi-directional laser scanning vectors were rotated Keywords: by 67° (Rot-scan) for each layer. The study highlights a correlation between laser scanning strategies with Selective laser melting resulting textures, microstructure, and mechanical properties in as-printed bulk 316L SS. The hardness, Young's Texture modulus, and ultimate tensile strength were significantly influenced by the final microstructure, crystallographic Austenitic stainless steel texture, and porosity. Furthermore, the applied laser scanning strategies made it possible to tailor crystallo- Solidification microstructures graphic textures locally within the component. This was demonstrated by printing characters with a fiber texture, in a matrix with ⟨100⟩ texture parallel to the Z-axis. © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction Additive manufacturing (AM) or 3D printing has become an indis- fi ⁎ Corresponding author. pensable tool to fabricate complex geometries, which are often dif cult E-mail address: [email protected] (J.J. Marattukalam). to manufacture by conventional techniques. The process offers cost https://doi.org/10.1016/j.matdes.2020.108852 0264-1275/© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 2 J.J. Marattukalam et al. / Materials and Design 193 (2020) 108852 reduction, product and process redesign, lead time reduction, and flex- ibility to manufacture parts with desired material properties [1–5]. Ad- ditive manufacturing has evolved from a mere prototyping and tooling technique to a direct part manufacturing method over the past few years [6]. A wide variety of materials have been processed through AM, including titanium, stainless steel, aluminum, nickel-based superal- loys, etc. [7–10]. Selective laser melting (SLM) is an AM technique where a scanning laser is used to melt along a certain direction in the powder bed. Typi- cally, high cooling rates and high thermal gradients are achieved in the SLM process, which results in a highly anisotropic solidification of the melt pool [11]. The thermal gradient and solidification front velocity can be used to predict the microstructural growth and morphology, while the cooling rate determines the size of the dendrites formed. Pro- cess parameters such as laser power, scan speed, hatch spacing, scanning strategies, and even sample orientation in the build plate have been reported to significantly alter the crystallographic texture during the process [12–15]. As crystalline texture is known to affect, Fig. 1. A secondary electron image showing the morphology of gas atomized 316L SS powder. e.g., mechanical and corrosion properties of a component, its control can be used to manufacture parts with desired material properties [16]. Several authors have discussed the impact of laser scanning strategy on the obtained texture in Ni-based alloys. For example, unidirec- Table S1. The printing was carried out in an EOS M100 (EOS GmBH, fi tional laser scanning was found to result in a fiber texture, while Germany) system equipped with a Yb ber laser. Different parameters bidirectional scanning of the laser gave rise to rotated cube-like texture including laser power, scan speed, hatch spacing, and hatch overlap, in the printed components [17]. Furthermore, a switching from trans- were varied in order to produce dense and crack free samples. The sam- verse anisotropic to isotropic and vice-versa has been demonstrated in ples used to study the effect of different scanning strategies were proc- the SLM of IN738 [18]. Crystallographic texture control in nickel-base essed using the optimised parameters, with a laser power of 107 W, μ superalloy Inconel 718 was demonstrated by varying the energy density scan speed of 800 mm/s, and hatch spacing of 70 m, to obtain the μ input to the powder bed using different electron beam scan strategies to best microstructural features. The thickness was set to 20 m for each μ locally change the grain orientation at precise locations in the as-build powder layer deposition. The laser spot diameter was 40 m, and bi- fl component [19]. A strong influence of laser scanning strategy on texture directional laser scanning was used. The process chamber was ushed has also been observed when processing wide range of other metals and with argon to minimize the presence of impurities in the material. alloys [20–22]. Three different scanning strategies were used in the printed parts, as One of the most common alloys is 316L SS, on which influences of shown in Fig. 2, Z-scan (along the raking direction of the powder bed), scan strategy on texture have been reported by Wei et al. [23]. They Y-scan (perpendicular to the raking direction), and 67° rot-scan with bi- showed that orientation of the primary dendrites was at 60° for all de- directional laser scanning vectors. The 3D computer-aided design (CAD) posited layers with unidirectional scanning laser scan, and for bi- models for the tensile samples were generated using Magics software, directional laser scan, it was found to be 90° in the neighboring layers. from Materialise, which is then numerically sliced into a number of Furthermore, in a recent study by Andreau et al. the effect of gas flow layers across the cross-section using EOS RP Tools 1.6 software. These and direction of laser scanning on crystallographic texture in 316L SS sliced layers are then recreated sequentially on a carbon steel build was discussed [24]. They observed that the crystallographic texture ap- plate to fabricate the components using an EOS PRINT software. peared to be dependent on the relative direction of laser scanning and The samples were cut (XY plane) and the pieces were polished using fi gas flow, which was attributed to an absorbing effect of the metal a series of SiC emery papers, diamond paste, and nally, silicon dioxide fi vapor plume. Scanning in the same direction as the gas flow was to obtain a mirror nish. The samples were etched in diluted aqua regia found to affect the depth of the melt pool, due to local attenuation of for the 10 s to reveal the melt pool patterns, which were observed using the laser beam's effective energy density transmitted to the powder Olympus AX70 optical microscope. The relative densities of as-printed bed. samples were determined using the Archimedes principle. The weight The aim of this work is to demonstrate how laser scanning strategy of the sample (W1) was measured in air at room temperature (RT). can be used to control texture during SLM, and even obtaining single The samples were then immersed in water and the weight (W2) was crystals of 316L SS. The microstructure and crystallographic textures measured at RT. The relative density of the sample was calculated as fol- formed during the process are correlated to the mechanical properties. lows; The use of laser scanning strategies to control the final shape of the ðÞ part and to simultaneously imprint a predesigned spatially resolved ðÞ¼% W1 x density of water Â : ð Þ Relative density ðÞ− : 100 1 crystallographic grain orientation pattern during the SLM process is W1 W2 x798 also demonstrated. The density of water at RT was taken to be 0.9978 g/cm3. The theo- 2. Materials and methods retical density of 316L SS is taken as 7.98 g/cm3. The weight of the samples was measured using an electronic balance with 0.1 mg accuracy. 316L SS gas atomized powders are produced by EOS, GmBH using Electron backscatter diffraction (EBSD) mapping was carried out industrial-grade feedstock materials. The as-received powder particles using an acceleration voltage of 20 kV and a beam current of 10 nA at were sieved using a 63 μm meshed sieve to remove oversized particles. 14 mm working distance in a Zeiss Merlin scanning electron microscope The sieved powder consists mostly of spherical particles, as shown in (SEM).

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